Not Applicable.
Not Applicable.
The present invention relates to the field of services for the petroleum and related industries and in particular to techniques for constructing and repairing oil wells, gas wells, geothermal wells, and the like.
An oil well or the like is cemented, in particular to isolate the well from the various geological strata it passes through and to prevent fluids from migrating between the various geological strata or between the strata and the surface. Such isolation seeks in particular to prevent gases from rising towards the surface up the annular space surrounding the casing which serves to maintain well integrity. Another purpose of cementing is to avoid oil-bearing zones being invaded by brines, or to prevent reserves of fresh water being contaminated by oil or by brine. So-called xe2x80x9crepairxe2x80x9d cementing is generally for the purpose of re-establishing sealing that has been lost due to the primary cementing deteriorating.
Underground reservoirs, which may contain hydrocarbons, brine, fresh water, or other fluids, are very frequently isolated from one another by fine layers of compact, leakproof clay. To ensure that cementing re-establishes such sealing, it is of great importance that adhesion to the walls of the well should be very good without any cracks appearing.
While a well is being constructed, it is common practice to use a drilling mud that is stabilized by clays such as bentonite, for example. Mud is a slightly jelled fluid, it deposits on the walls and on centralizers, or, on porous walls, generally constituted by geological formations, it constitutes a mud or filter cake. Such cakes are compressible, and under the effect of pressure and of mud flow, they are transformed into a very compact layer which can itself constitute an acceptable interface between the underground formation and cement. Nevertheless, as the thickness of the mud cake increases, the porosity of the effective wall (cake plus geological formation) decreases and the cake which continues to be deposited becomes much less compact until it constitutes a xe2x80x9csoft mud cakexe2x80x9d made up of water-swollen clays. Cakes that are thick and soft tend to form in formations that are very permeable, such as unconsolidated coarse sands. Jelled mud deposits also occur in zones of low mud flow, e.g. level with centralizers, or, if the casing is off-center, in the narrower portion of the annulus between the casing and the underground formation.
Such soft cake is often the cause of subsequent leakage, and as a result efforts are made to eliminate it as much as possible. That is the function of cleaning fluids commonly known as xe2x80x9cspacersxe2x80x9d or as xe2x80x9cchemical washesxe2x80x9d. As a general rule, such cleaning fluids contain detergents which clean the casing and the geological formations through which the borehole passes. The detergents are generally of the surface-active type adapted to the nature of the mud used, and in particular to the nature of the continuous phase of the mud (water or oil). They tend to favor swelling and dispersion of soft deposits so as to make them easier to eliminate by fluid flow.
When the geological formation contains clays, mud cake does not form in contact therewith because the clays are impermeable. As a result, the spacers cause the reactive clays of such formations to swell. xe2x80x9cCleaningxe2x80x9d the well then becomes locally more harmful than favorable.
When cementing to repair sealing, even though the fluid preceding or following the cement is generally not mud, the problem is very similar. Clay-containing mud may have remained, and as a result of the presence of underground water, reactive clays will have swelled. The presence of swollen clays prior to cementing is bad for sealing since contact between cement and swollen clay is poor.
The present invention improves placement of cement slurries in wells in geological zones containing swelling clays or mud residues containing clays. According to the invention, the well zone thought to contain water swelling clays is treated with a fluid containing clay precipitating agents capable of coagulating the clay prior to the cement slurry being put into place in the well during the cementing stage.